Automotive accessories and light industrial controls often incorporate small, reversible, two terminal DC motors and other loads, which must be controlled from locations some distance away from the load. In these applications it is desirable to control load operation relatively simply and inexpensively, using minimal parts and keeping the quantity of wires running to the load to a minimum.
In automotive applications it is especially critical that the quantity and size of wires run, be kept to a minimum. Typically, the wires distributed to control various loads in locations throughout an automobile are "bundled" and distributed in "wire harnesses." The proliferation of electrically controlled automotive options has resulted in wire harness bundles of considerable magnitude. Because of the need to package wire harnesses in vehicles, out of the view of vehicle occupants, serious consideration is given to reducing bundle sizes.
In order to package wire harnesses in the infrastructure of a vehicle, they are typically installed early in the auto assembly process. Manufacturers may install wires in wire harnesses which distribute wires throughout a vehicle for options that are not installed in all vehicles by the manufacturer at the time of assembly. These "give-away wires" facilitate the installation of options and reduce the number of different wire harness types required. Wire harnesses with give-away wires benefit manufacturers by providing flexibility to install accessories. However manufacturers desire to minimize give-away wires in order to reduce product cost.
Control of accessory loads, such as two terminal reversible DC motors used to actuate power windows, power door locks, power seats, power mirrors or the like, normally involves running at least two power wires and multiple control lines to the two terminal reversible motors and related circuitry. The wires facilitate motor operation in a forward or clockwise mode by directing current flow to a positive motor terminal and in a reverse or counterclockwise mode by directing current flow to a negative motor terminal. A third mode that is often desirable, dynamic braking, may be obtained by shorting the positive and negative motor terminals together.
Several methods of providing three-state motor control (forward or clockwise rotation, reverse or counterclockwise rotation and dynamic braking), for two terminal motors exist. A direct high-current control method involves switching a source voltage at a complex high current switch and delivering current to one motor terminal or the other via two high-current wires. System expansion, by adding additional motors, requires one additional high-current switch and two additional high-current wires for each added motor. The complex switching and multiple high-current wires running to the motors present drawbacks in this method, especially when additional switches are desired to control the same motor from several locations. For instance, controlling a power window motor from the drivers door and a remote door in which the window resides, with this method, would require two high-current switches and four high-current wires for the one motor.
Simple low-current switching can be used by controlling relays to effect three-state motor control. This method, embodied in U. S. Pat No. 4,288,726 improves on the direct high-current control method by eliminating the need for complex high-current switches. Three high-current wires are used to provide power to several motors. Only one relay per motor is required. System expansion by adding new motors requires only a low-current switch and one relay for each motor added, however, a serious limitation in simultaneous motor operation exists in this method, in that any motor selected must simultaneously be commanded to rotate in the same direction.
A relatively sophisticated electronic communication method uses only one pair of high-current wires to power several motors. Electronic encoder/decoder circuitry facilitates relay selection. Only a single low-current switch and one relay needs to be added for each motor added to expand the system, however, the reduction in wires run may not justify the expense of the electronics needed to encode and decode serial communication signals. Similarly, a failure in the communication line or in the encoding and decoding electronics may disable the entire system.
Various, similar multi-wire switching control configurations also exist for controlling non-motor loads requiring three-states of operation. As with motor loads, other DC loads such as lighting circuits, normally require at least two power wires and multiple control lines.